Prometheus exporter for NGINX Unit metrics

[lcrilly]

The NGINX Unit control API includes a /status endpoint for usage statistics. Here is a solution for exposing these metrics in Prometheus format:

https://gist.github.com/lcrilly/ad516de8378dd8ae5303a942e67d55b5

[Pavlusha311245]

In addition to the discussion, I will attach a link to this issue #837 and also make my suggestions.
It might be worth making an endpoint/metric and also adding the ability to disable it, for example when running unitd --prometheus as an argument